1. Field of the invention
Present invention relates to a focus detecting device for a camera, particularly a focus detecting device which detects a focus condition of a camera objective lens in accordance with the output of a charge accumulation type solid state image sensor which receives the image of a target object formed thereon through the objective lens.
2. Description of the Prior Art
A charge accumulation type solid state image sensor such as a CCD (charge coupled device) is composed of a photoelectric converter section for converting the intensity of light incident thereon into an amount of photoelectric charge, a charge accumulation or charge integration section for integrating the converted charge at a rate in accordance with the intensity of the incident light, and a transfer section for transferring the integrated charge as an image signal to an outer circuit. Hence, the integration time required for integrating the charge from zero to a given level depends on the intensity of the incident light, and increases as the intensity of the incident light weakens. In other words, more time is required for the detection of the focus condition as the intensity of the incident light weakens, because it is impossible to detect the focus condition in accordance with the image signal from the image sensor when the amount of the integrated charge is too low.
In a camera which detects the focus condition in accordance with the image signal supplied from such charge integration type image sensor, it is likely to occur that the shaking of a hand-held camera affects the focus detection as the detection takes longer time. When the intensity of the incident light is low, the camera-to-object distance is likely to change and the object image is likely to shake or vibrate on the image sensor during the focus detection, thereby disabling the accurate detection of the focus condition. Additionally, the longer focus detection time sometimes causes missing of exact shutter actuation and disables the focus detecting device from responding to a moving object.
As a countermeasure to cope with the above problems, a U.S. patent application Ser. No. 669,107 assigned to the same assignee as the present application, discloses a focus detecting device wherein the integration time of the image sensor is restricted. When the integration time is expected to be longer than the limit, a fixed time is given for the integration, with the voltage corresponding to the charge integrated during the fixed time being amplified to compensate for the shortage of the integration time. In the focus detecting device of the aforementioned U.S. patent application, the integration time of the image sensor is restricted or limited to 200 m sec and the image signal is multiplied by 2, 4 or 8, e.g. doubled if the integration time is expected to be 400 m sec without restriction, so that a signal is equivalent to the signal to be obtained by the integration for 400 m sec. The integration limit time is decided or designed to be an allowable maximum time for the focus detection. Hence, the problems as discussed above, remain or are not solved sufficiently for the case when the integration time is shorter than the maximum time but considerably long. This is because the integration limit time should be long enough to guarantee a sufficient amount of charge to be integrated by the image sensor. Otherwise, the image signal is too low to be used for the focus detection or the S/N ratio of the signal will be too low to derive the image signal. Thus, with the device proposed by the U.S. patent application, there exists a range wherein the integration time is shorter than the limit but so long that an exact or desired shutter actuation is likely to be missed or that the focus detecting device can not follow or respond to the movement of an object. In such a range, the integration time depends on the light intensity on the image sensor and is not restricted.
Additionally, the focus detecting device of the U.S. patent application is designed only to multiply the level of the image signal by 2, 4 or 8 when the light intensity on the image sensor is low and a fixed time is given for the integration. Hence, focus detection can not be made when the brightness and contrast of an object are so low that it is difficult to discriminate the image signal from noise signals or that even the multiple signals do not suffice for the focus detection.